Why is an assignment to a base class valid, but an assignment to a derived class a compilation error?

Question

This was an interview question. Consider the following:

struct A {}; 
struct B : A {}; 
A a; 
B b; 
a = b;
b = a; 

Why does b = a;

Answer 1

If I am getting interviewed then, I will explain in little philosophical way.

a = b;

is valid, because every B contains A as its part. So a can extract A from within B. However, A doesn't contain B. thus b cannot find B from within A; that's why,

b = a;

is invalid.

[Analogically, a void* can be found in any Type*, but Type* cannot be found in void* (thus we need a cast).]